The invention relates to a process for preparing N,N-substituted 1,3-propanediamines.
N,N-Substituted 1,3-propanediamines are obtained typically by reacting secondary amines with acrylonitrile and subsequently hydrogenating the nitrile group to the amine. For example, the industrially significant N,N-dimethyl-1,3-propanediamine (DMAPA) is prepared on the basis of dimethylamine and acrylonitrile. The acrylonitrile starting material is typically obtained by the SOHIO process, a catalytically controlled synthesis of propene with ammonia and elemental oxygen. This forms acrylonitrile with elimination of water, with acetonitrile and hydrogen cyanide as by-products.
Access to chemical commodity products such as propene from renewable raw materials is generally uneconomic, since the complex molecular structures formed by nature with considerable energy expenditure can be reshaped to low molecular weight substances only with expenditure of energy.
A low molecular weight chemical commodity product which, however, can be obtained commercially from renewable raw materials is acrolein, which is prepared by dehydrating glycerol. Glycerol in turn is obtained typically as a by-product in the conversion of fats and oils to fatty acids (fat hydrolysis) or fatty acid methyl esters (biodiesel).
The reaction of secondary aliphatic amines with acrolein was described for the first time by Mannich et al. (C. Mannich, K. Handke and K. Roth, Chem. Ber., 69, 2112 (1936)). Reaction of dimethylamine with acrolein in the presence of a dehydrating agent or in a water-immiscible solvent afforded, within a narrow pressure and temperature range, N,N,N′,N′-tetramethyl-1,3-propenediamine, which was converted further in a further stage by catalytic reduction with hydrogen over platinum oxide and cyclohexane as a solvent to the corresponding N,N,N′,N′-tetramethyl-1,3-propanediamine. According to the disclosure, the water which forms in the reaction has to be removed from the reaction mixture since the products which form are readily hydrolyzable. Some of the N,N,N′,N′-tetramethyl-1,3-propanediamines obtained by the reaction described by Mannich et al. are used as catalysts in the preparation of polyurethanes.
A process for reacting primary aliphatic or cycloaliphatic amines with acrolein to give N,N′-substituted 1,3-propenediamines is likewise disclosed in U.S. Pat. No. 2,565,488 and in the equivalent DE-B-866647. The amounts of water which form in the reaction can, according to the disclosure, be left in the reaction mixture. When the reaction, however, is performed in the presence of dehydrating agents (desiccants), higher yields of N,N′-substituted 1,3-propenediamine are achieved. It is stated that N,N′-substituted 1,3-propenediamines can be saturated with hydrogen, so as to obtain, for example, N,N′-substituted 1,3-propanediamines. It is also stated that N,N′-substituted 1,3-propenediamines can be transaminated with amines which comprise other substituents in a further reaction step before the hydrogenation.
DE-A1-4232424 relates to a process for preparing N,N′-substituted or N,N,N′,N′-substituted unsaturated amines by reacting 2-alkenals with primary or secondary amines at temperatures of from (−20) to 70° C. and pressures of from 0.10 to 100 bar, by performing the reaction without removing the water of reaction. In addition, the disclosure describes the transamination of the unsaturated amines thus obtained by the reaction thereof with other primary or secondary amines at temperatures of from 20 to 150° C. The unsaturated amines obtained in DE-A1-4232424 can, according to the disclosure, be hydrogenated with hydrogen at a pressure of from 1 to 350 bar and temperatures of from 0 to 150° C., using, for example, palladium supported on activated carbon as the hydrogenation catalyst.
Finch et al. (H. D. Finch, E. A. Peterson and S. A. Ballard, J. Am. Chem. Soc., 74, 2016 (1952)) describe the reaction of acrolein or methacrolein with primary or secondary amines. The N,N′-substituted 1,3-propenediamines or N,N,N′,N′-substituted 1,3-propenediamines thus obtained are, according to the disclosure, hydrogenated in a subsequent stage to the corresponding saturated N,N′-substituted or N,N,N′,N′-substituted 1,3-propanediamines, or heated together with other amines, such that an amine exchange takes place. It is stated that amine exchange and hydrogenation can also be carried out simultaneously in the presence of a Raney® nickel catalyst. For instance, N-isopropyl-1,3-propanediamine was obtained in two stages, the first stage involving initial reaction of acrolein with isopropylamine and the second stage hydrogenation of the resulting reaction mixture, after removal of excess amine and solvent, with ammonia in the presence of Raney® nickel. The yields of N-isopropyl-1,3-propanediamine, based on the acrolein used in the reaction, were less than 68%.
The problem of the present invention consisted in providing a process for preparing N,N-substituted 1,3-propanediamines from acrolein, the intention having been to achieve a higher selectivity based on the acrolein used as compared with the prior art. More particularly, it was an object of the invention to reduce the formation of by-products, such as 3-methylpiperidine and N,N,N′,N′-substituted 1,3-propanediamines, which can form in the hydrogenation of the N,N,N′,N′-substituted 1,3-propenediamines which form as an intermediate. It was a further aim to provide a process in which the N,N,N′,N′-substituted 1,3-propenediamines obtained as an intermediate need not be isolated or purified before the further conversion to N,N-substituted 1,3-propanediamine. The intention was thus to achieve an easy to handle process which can be implemented in a technically simple manner. More particularly, it was an aim of the present invention to provide a new preparation route for DMAPA, in which feedstocks which can be obtained on the basis of renewable raw materials are used.